The present invention relates to a method for preventing biofouling of surfaces continuously in contact with sea water or, more particularly, to a method for preventing the microbiofouling film formed by the proliferation of bacteria in the sea water on the surface of various facilities continously in contact with sea water to cause drawbacks such as decrease in the heat transfer coefficient through the surface.
As is well known, sea water is utilized in large volumes mainly as a cooling medium or with an object of heat recovery in heat exchangers in various facilities such as power stations and chemical plants located at seaside as well as in the evaporators of LNG in LNG import bases and engines of vessels. In recent years, intensive development works are under way on the ocean thermal energy conversion system as a new energy source utilizing the temperature difference of sea water between the surface layer and the depth of ocean. In such a system, however, the utilizable temperature difference cannot exceed about 20.degree. C. so that one of the key problems in this technology is how to ensure the highest possible efficiency of heat exchange. This is because the surfaces of heat exchangers continuously in contact with the sea water intake are unavoidably covered with a microbiofouling film formed by the proliferation of microorganisms in the sea water to cause serious decrease in the heat transfer coefficient through the surface.
Of course, various methods have been proposed and are practiced to solve this difficult problem. One of the approaches for the purpose is a chemical means in which the heat transfer surface is periodically sterilized to destroy the adhering biofouling film by use of various anti-fouling chemical substances having toxicity to the fouling microorganisms including, for example, chlorine, bromine, bromine chloride, hydrogen peroxide, ozone, permanganates, arsenates, arsenites, cyano compounds, heavy metal salts, organometallic compounds, phenols and the like. Such a method of using an anti-fouling chemical agent is, however, not quite satisfactory and the applicability of the method is under strict limitations even by setting aside the problem of expensiveness of such a chemical agent because the anti-fouling agents are also toxic to other harmless microorganisms as well as any oceanic living organisms in general to cause a serious problem of environmental disruption.
Alternatively to the above mentioned chemical means, several physical and mechanical methods have been proposed for preventing biofouling of surfaces in sea water including the hot-water treatment method, osmotic impact method, ultraviolet irradiation method, ultrasonic vibration method and mechanical scrubbing method using sponge balls or brushes. Each of these methods, however, is defective in one or more respects and far from satisfactory. For example, the applicability of the method of hot-water treatment is limited to the plant sites where a large volume of hot waste water is available because otherwise the expenses for the facility investment and energy cost for the hot water production are almost prohibitive of the method. The applicability of the osmotic impact method is under limitation by the availability of a large volume of fresh water and also by the disadvantage in the working efficiency that the method can be performed only with interruption of running of heat exchangers and other facilities. Further, the effectiveness of the methods of ultraviolet irradiation and ultrasonic vibration is suddenly decreased as the distance from the ultraviolet or ultrasonic apparatus is increased so that no satisfactory results can be obtained unless a number of such apparatuses are installed with great expenses. The method of mechanical scrubbing requires certain specialty apparatus and can be performed also only with interruption of running of the heat exchangers in addition to the serious problem of possible damages caused on the surface under mechanical scrubbing as well as incompleteness of scrubbing for the removal of the biofouling films especially when the scrubbed surface has a complicated form.
Thus, none of the prior art methods is quite satisfactory for preventing biofouling of surfaces in contact with sea water due to the above described problems and disadvantages encountered in practicing the method so that it has been eagerly desired to develop a novel and improved method for the purpose freed from the problems and disadvantages in the prior art methods.